Experience from subsea drilling operations in upper soil layers has shown that the subsurface formations to be drilled usually have very low fracture strength (301) close to the seabed and it is often close to that of seawater (302). This dictates that drilled formation will have to be disposed on seabed since the formation strength is not high enough to support the hydrostatic pressure from the combined effect of drilling mud and the suspended drilled formation solids in a drilling riser up to the drilling platform (304). This is the reason for that it is not possible to install a conventional drilling riser and take the returns to the surface, before a casing is set so deep that it will isolate the weaker formation and that the soil strength is high enough to support a liquid column of water and formation cuttings (debris) up to the drilling unit above sea level.
The 2 uppermost sections of the hole are normally drilled riserless, without a drilling riser. Often this “pump and dump” procedure cause for excessive amount of drilling mud, barite weighting materials, formation solids and other chemicals to be dumped to the ocean. Besides this practice being expensive it is also a wasteful process that can be harmful to marine life on the ocean floor.
In deeper waters as the hole deepens, the difference between the formation pore pressure and the formation fracture pressure remains low. The fracture gradient is so low that it can not support the hydrostatic pressure from a full column of seawater and formation cuttings up to the drilling platform. In addition to the static hydraulic pressure acting on the formation from a standing column of fluid in the well bore there are also the dynamic pressures created when circulating fluid through the drill bit. These dynamic pressures acting on the bottom of the hole are created when drill fluid is pumped through the drill bit and up the annulus between the drill string and formation. The magnitude of these forces depends on several factors such as the rheology of the fluid, the velocity of the fluid being pumped up the annulus, drilling speed and the characteristics of the well bore/hole. Particularly for smaller diameter hole sizes these additional dynamic forces can become significant. Presently these forces are controlled by drilling relatively large holes thereby keeping the annular velocity of the drilling fluid low and by adjusting the rheology of the drilling fluid. The formula for calculating these dynamic pressures is stated in the following detailed description. This new pressure seen by the formation in the bottom of the hole caused by the drilling process is often referred to as Equivalent Circulating Density (ECD).
Since this ECD effect can be neutralized by the system as described in patent application PCT/NO02/00317 the surface hole can be drilled deeper than with conventional drilling methods. This is an advantage since the next section can also be drilled deeper hence it is possible to the drill the well with fewer casings if the surface casing can be set deeper. Hence considerable economic effects can be expected from drilling the surface hole deeper.